1. Field of the Invention
The present invention relates to a motor controller which controls a switched reluctance motor.
2. Description of the Related Art
The switched reluctance motor (hereinafter referred to as an SRM) is gaining attention for its low manufacturing cost and simple yet robust structure. The rising price of rare-earth elements is also contributing to the popularity of the SRM which requires no rare-earth permanent magnet. In the SRM, torque is obtained by applying a current to the stator winding while the inductance is changing.
In the SRM, regardless of its control method, the variation width of a magnetic flux becomes smaller during high-speed rotation, which results in a torque decrease. One example of a method for suppressing torque decrease is a control method disclosed in Japanese Patent Laid-Open No. 2003-189669, which involves a continuous current mode (hereinafter referred to as a CCM).
In the CCM, the phase windings are energized before the magnetic flux and the current return to zero at the end of the energy return period. The phase windings are always linked by the magnetic flux caused by the current continuously flowing through them. To realize the CCM, the electrical angle at which the phase windings are excited is enlarged. Accordingly, the magnitudes of the current and the magnetic flux are raised, so that these do not return to zero. That is, the next excitation occurs before the current becomes zero. As a result, even during high-speed rotation, a constant variation width of the magnetic flux is maintained and torque decrease is prevented.
In the CCM, it is necessary to acquire the accurate rotational position of the rotor in order to enlarge the electrical angle at which the phase windings are excited. If there is a gap between the acquired rotational position and the actual rotational position of the rotor, excitation fails to be started at a proper timing, so that no ideal continuous current flows and the motor efficiency degrades.